1. Field of the Invention
This invention relates generally to compression ignition engines, and, more particularly, relates to a method and apparatus for controlling a gas excess air ratio in a compression ignition natural gas engine.
2. Discussion of the Related Art
Recent years have seen an increased demand for the use of gaseous fuels as fuel source in internal combustion engines. Gaseous fuels such as propane or natural gas are considered by many to be superior to diesel fuel and the like as a dual source for compression ignition engines because gaseous fuels are generally less expensive, provide equal or greater power with equal or better mileage, and produce significantly lower emissions. This last benefit renders gaseous fuels particularly attractive because recently enacted and pending worldwide regulations may tend to prohibit the use of diesel fuel in many engines. In addition, adapting an engine to be fueled at least in part by gaseous fuels can significantly reduce an engine's carbon footprint, particularly if the gaseous fuel is obtained from biomass or another carbon-neutral source. The attractiveness of gaseous fuels is further enhanced by the fact that existing compression ignition engine designs can be readily adapted to burn gaseous fuels.
When used to fuel compression ignition engines, the relatively compressible gaseous fuel typically is ignited through the autoignation of a “pilot charge” of a relatively incompressible fuel, such as diesel fuel, that is better capable of compression ignition.
Lean burn engines, including standard diesel engines and dual fuel engines, have a wide range of desired lambdas as compared to a gasoline engine which generally operates in a small band around the stoichiometric (lambda=1). To improve performance, some lean burn engines have relied on open loop lambda control using empirical data obtained during system development. Such systems control fuel and/or air supply (such as through exhaust gas recirculation (EGR) or turbo wastegate control) to achieve or maintain an experimentally determined ideal lambda for prevailing speed and load conditions.
However, gaseous fuels have a relatively narrow range of useful excess air ratios or lambdas (defined as the ratio of total air available for combustion to that required to burn all of the fuel). In any fuel, if lambda drops below a minimum threshold, NOx and other emissions increase to unacceptable levels. On the other hand, if lambda rises above a maximum acceptable threshold, misfiring can occur, resulting in excessive unwanted emissions and sharply decreased thermal efficiency.
It is therefore essential for optimum control of combustion in gas fueled engines to maintain lambda values within a permissible range, and preferably to cause lambda values to approach optimum levels. This control is hindered by the fact that engine performance and exhaust emissions may change over time and/or may not correlate precisely with pre-calibrated characteristics when the engine is operated in the field under varying operating conditions. As a result, given air and fuel supplies and a given EGR ratio may not achieve a predetermined lambda at prevailing engine operation conditions.
This problem could be alleviated through closed loop lambda control using EGO (EGO) concentration as a feedback, it being recognized that EGO concentration correlates directly to lambda. However, closed loop lambda control based on desired EGO concentration is complicated by a variety of factors. The desired EGO concentration can change significantly depending on prevailing operating conditions, fuel quality, and other factors affecting fuel and air supply. Lambda variations and variations in combustion efficiency also hinder the determination of a desired EGO concentration. In addition, even if the desired EGO content can be precisely calculated, the lag between the generation of the fuel demand signal and the resultant EGO concentration determination hinders real-time feedback of lambda control using EGO concentration measurements.
The need therefore has arisen to provide lambda control in gaseous fueled compression ignition engines using a closed loop feedback in view of the variations in operating conditions and fuel quality, and in view of limitations imposed by feedback loop timing.